Nongravimetric mass determination system

ABSTRACT

A NON-GRAVIMETRIC MASS MEASUREMENT SYSTEM HAVING A SUPPORT STRUCTURE FOR HOLDING A MASS TO BE &#34;WEIGHTED&#34; ATTACHED TO AN OSCILLATING SPRING ASSEMBLY. A DEVICE FOR INDICATING PRECISELY WHEN THE OSCILLATING MASS CROSSES POINT OF ZERO DISPLACEMENT PRODUCES A SIGNAL WHICH IS SENT TO A COUNTER FOR DETERMINING THE TIME PERIOD. A SUBSTANTIALLY FRICTIONLESS AIR BEARING SERVES TO RESTRAIN THE MOTION OF THE OSCILLATING MASS TO A SINGLE AXIS UNDER ZERO GRAVITY CONDITIONS IN SPACE WHILE SERVING TO SUPPORT THE WEIGHT ON EARTH.

U I 5 W 4 Pr Jan. 1971 w. E. THORNTON 55 6 Filed May 20, 1968 3Sheets-me l NONGRAVIMETRIC MASS DETERMINATION SYSTEM INVENTOR. rid/WWOBY ZZZZ" 1971 w. E. THORNTON 3555586 NONGRAVIMETRIC MASS DETERMINATIONSYSTEM Filed May 20 1968 3 Sheets-Sheet 2 INVENTOR. M4444 m/a/P/s m/v BYZZZ JW I774 KEYS 155711 w. E. THORNTON NONGRAVIMETRIC MASS DETERMINATIONSYSTEM 3 Sheets-Sheet 5 Filed May 20 1968 INVENTOR. Mid A4", WOW/Y.ilaited States 3,555,886 NONGRAVIMETRIC MASS DETERMINATION SYSTEMWilliam E. Thornton, San Antonio, Tex., assignor to the United States ofAmerica as represented by the Secretary of the Air Force Filed May 20,1968, Ser. No. 730,461

Int. Cl. Gillg 3/16 US. Cl. 73-673 4 Claims ABSTRACT OF THE DISCLOSURE Anon-gravimetric mass measurement system having .a support structure forholding a mass to be weighed BACKGROUND OF THE INVENTION This inventionrelates to a means for the determination of mass independently ofgravitational forces and, more particularly, the invention is concernedwith providing a precise means for determining the period of oscillationof an oscillating mass and linear spring arrangement which varies inproportion to the change in mass.

One of the problems encountered by man as he extends his ability to stayin outer space for longer time periods is the effect of long termexposure of the human body to zero gravity conditions. It has been foundthat there generally occurs a loss of body weight which corresponds tothe length of time in the space environment. This weight change may becaused by loss of fluid by diuresis during the first days ofweightlessness and by atrophy resulting from the decreased use ofcertain muscles during the long periods spent in the confiningconditions in space capsules. Exercise procedures are necessary tocounteract loss of muscle mass and to return the body exercise level tonormal in order to prevent this disuse atrophy. The change in weight ofthe human body in the space environment is the most valuable indicatorof the physical condition of the astronaut. Thus, an accurate andprecise system for measuring the body mass at zero gravity is needed todetermine the effectiveness of exercise procedures which are introducedto counteract loss of muscle mass and to control intake of fluids.

SUMMARY OF THE INVENTION The present invention is primarily concernedwith the provision of a system for accurately determining the mass andcorresponding equivalent earth weight of various objects in spaceincluding the weight of the human body. In the practice of the inventionthe man or object whose mass is to be determined is rigidly attached toa supporting structure which allows linear motion along only one axis.Elastic restoring forces are applied to the mass and supportingstructure such that when it is displaced and released, simple, virtuallyundamped harmonic motion ensures. A device to determine precisely whenthe oscillating mass crosses the point of zero displacement and acounter for determining the time period of the oscilating mass areincluded in the system. Also, some means for providing displacementalong a given axis and a bea'ringto restrain the motion to this axisonly are required. The bearing is capable of supporting the weight atentice on earth while operating in space under zero gravity conditionswithout adding appreciable friction.

Accordingly, it is an object of the invention to provide a system forprecisely determining the change in mass and corresponding change inweight of an object such as a living human body under conditions ofweightlessness encountered in outer space.

Another object of the invention is to provide a system for determiningthe change in mass of an object by measuring the oscillating frequencyvariation of the object and the supporting structure upon which it ismounted.

Still another object of the invention is to provide a mass measuringsystem suitable for use under non-gravitational conditions wherein anoscillating spring-mass and counter arrangement is utilized to achieve adetermination of the change in mass based upon the period of oscillationwhich varies as a function of mass.

A further object of the invention is to provide a mass measurementsystem which allows true translatory motion and yields the ultimate inaccuracy. Weight measurement and/or weight changes on the order ofi0.1.% can be determined by use of the present invention.

A still further object of the invention is to provide a mass measurementsystem which operates at extremely low amplitudes of oscillation. Azero-crossing detector which is sufiiciently sensitive to movements of afew micro-inches allows amplitudes of oscillation on the order of 0.2inch.

Another further object of the invention is to provide a mass measurementsystem in outer space which requires a smaller operating area because oflow amplitudes of oscillation and prevents the creation of G forces inthe spacecraft. Also, wind resistance and associated eflects which lendto the possibility of errors are eliminated by the low velocity andamplitude of the oscillations.

These and other objects, features and advai itages will become moreapparent after considering the description that follows and from thedrawings wherein like numbers are used throughout to identify likeelements.

DESCRIPTION OF THE DRAWINGS In the drawings: i; 1

FIG. 1 is a schematic illustration of a simple mass and spring system;

FIG. 2 is a schematic representation of the basic principle of thesystem for measurement of mass employed in the present invention;

FIG. 3 is a schematic illustration of the optical arrangement of thezero-crossing detector;

' FIG. 4 is a simplified zero-crossing detector circuit;

FIG. 5 is a view in side elevation of a modified crew v position seatfor use in mass determination;

FIG. 6 is a perspective view showing details of the mass measurementequipment of the crew seat of FIG. 5; and FIG. 7 is a schematicrepresentation of one exemplary instrumental arrangement of the massmeasurement system according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Virtually all of the massmeasurements made on earth are comparisons of forces produced by astandard and unknown masses attracted by the earth's ubiquitousgravitational field. In the absence of or compensation of this field,such as occurs in orbital or space flight, some other means must be usedto obtain the mass of unknown objects.

The present invention provides an accurate and precise system foraccomplishing mass determination and is based on the combination of themass into a linearly oscillating spring and mass pendulum. An idealizeddepiction of the system is shown in FIG. 1 wherein the mass, M, moves ina, all the X direction with the spring, K, and the resistance, R, forcesacting. The general equation for such a system is:

The solution of this equation in terms of the period (time) of oneoscillation is:

T 2 1r A! K where: T=time K=spring constant M=mass It is assumed that Rand an initial displacement in X was made and the mass released withoutfurther disturbance.

Under these conditions, mass may be determined from:

If the spring constant is not known, it may be determined by calibrationwith a known mass.

In a real system R f) for there will be resistance losses associatedwith the spring, air resistance, and with any bearing system which maybe required. These resistance losses will have two effects: (1) theperiod of oscillation willbe changed and (2) the oscillation amplitudewill be reduced in a logarithmic fashion. In an acceptable system, theresistance losses should be small enough to produce negligible errors inperiod when compared with other errors. The decrease in amplitude is ofno concern so long as the zero-crossing detector is placed at the pointof zero displacement.

Referring now to FIG. 2, a working system, according to the invention,includes a support structure 13 to hold the mass 15 to be weighed, aspring assembly 17, a device 19 to determine precisely when theoscillating mass crosses point of zero displacement, a counter 21 fordet'ermining time periods, some means 16, for example, a drum and cablewith a quick disconnect at 18, for providing displacement along a givenaxis, and a bearing 23 to restrain the motion to a single axis and onearth to support the weight without adding appreciable friction. Apractical realization of the scheme imposes many considerations of theoperation and arrangement as do the severe limitations imposed by spaceflight.

Some of these considerations are: Motion of the mass to be measuredshould be restricted to the minimum degrees of freedom or linear motionalong a single axis. This motion should be of low amplitude and velocityespecially in the case of non-rigid masses including man. This implies ahigh resolution zero-crossing detector.

The realization of such a mass measurement system is shown schematicallyin FIG. 2. A linear air bearing 23 supports the weight under gravity andrestricts the motion to a single axis. Such an air bearing provides adevice of ,low enough mechanical resistance to be used in such anapplication. The movable portion of the bearing also supports the massesto be measured. To this portion of the device a pair of precisionsprings 17 are attached in pullpull fashion. These springs must havecarefully controlled characteristics to obtain the required degree ofaccuracy. The zero-crossing detector 19 is an electrooptical arrangementwhich provides a distance resolution better than X10- inches withoutloading the bearing.

In FIG. 3, there is shown schematically a detailed illustration of thezero-crossing detector 19. An incandescent bulb 25 illuminates a slit inthe optic tube 27 assembly which is then focussed as a vertical ribbonof light .001 inch wide by approximately .1 inch high at X A knife edge29 is attached to the oscillating mass M and moves across the axis X. Atall points above X the photo electric cell 31 is illuminated fully andprovides a maximum light output. At X the light is cut off in .001 inchand remains 4 off when the mass is below this point. The system thus hasan inherent accuracy of .001 inch.

This basic accuracy of .001 inch is further enhanced by the electroniccircuitry shown in FIG. 4 as follows: The voltage e is the output of thephotocell while e is fixed at ,5 the maximum voltage of 2 A stable highgain differential amplifier 33 provides an output of approximately 20volts for a 1 mv. difference between e and e The slope of the photoelectric cell 31 output is several volts/ 10- inches as the knife edge29 crosses the light beam. This results in a theoretical resolution ofmicroinches. Practically there is some motion in other planes withdefocussing and other effects. The output of the zerocrossing detector19 must then be converted to signals controllihg the timer. The simpleR.C. differentiator shown will generate a pulse each time zero iscrossed. Since zero is crossed twice each cycle and from alternatelyopposite directions, only pulses of a single polarity will be passed tothe counter which will then count every other complete cycle.

The timer used may be a commercial unit with 1 microsecond resolution. Asear mechanism (not shown) releases the mass from a maximum displacementof inch for a total peak to peak amplitude of /1 inch. The release mustbe carefully performed to prevent introduction of transient forces intothe oscillator which would transiently alter the oscillation frequency.One arrangement which would produce the desired effect is a springloaded sear which is a face hardened and carefully ground wedge removedfrom a larger diameter, face hardened and ground circle attached to theoscillator. An air supply and regulator for the translatory air bearing23 completes the arrangement which is shown schematically in FIG. 7.

MODE OF OPERATION OF THE INVENTION Operation consists of releasing themass from a small displacement and automatically timing a number ofcycles. By taking the mode of several cycles, extraneous vibration andother error producing effects are reduced.

A somewhat more complex requirement for this system is measurement ofmans mass in space. The problem here is a rigid combination of theflexible body of man into an oscillating system. FIGS. 5 and 6 show anapparatus for investigation of these problems. It follows thearrangement shown and also includes a crew seat having a pair of handles35, foot board 37, and head rest 39 to allow contraction of the bodysmusculature both to stiffen the body and provide rigid attachment to thescale.

For practical utilization, this device must be capable of performanceunder space conditions. These conditions include severe limitations onsize, weight, power, complexity (particularly of operation) as well as avariety of environmental conditions including vibration and G loadingduring powered flight, sub-normal atmospheric pressure and possibleunusual atmospheric composition.

The oscillating mass scheme lends itself well to such conditions. Thecondition of weightlessness will be an asset. The only bearings requiredunder zero G are to restrain small forces from deviating the mass out ofthe desired axis of oscillation. A small pair of cylindrical airbearings 41 operating with 1-2 p.s.i. are all that should be required.The air flow rate will also be low such that a small bleed from a bottlecontaining atmospheric gases or a small pump would be needed for thebrief period of measurement. The reduced atmospheric pressure with itslowered viscosity will also be an asset.

In the case of mass determination of the crew, a modification of anormal crew position seat may be used as shown in FIGS. 5 and 6. Duringlift off and normal periods, the movable portion of the seat will beclamped to the rigid structure 43 contiguous with the ship by means ofthe handle 45. The spring assembly on the back which is positionedbetween the seat and rigid structure 43 operates to suspend the seat onthe rigid structure 43 and will also be unloaded by the handle 45. Formass determination, with the handle 45 in a clamped position, the springwill be placed in tension and when the seat is unclamped by release ofthe handle 45 it is allowed to be displaced upward a short distance. Thecrew member will grasp the side handles 35 and exert pressure againstthe foot board 37 to provide body rigidity and firm attachment to theseat.

A small, less than one inch, displacement will be made and a few cyclesof oscillation will take place along the axis of the twin air bearings41 restraining seat to ship. An optical pickoff will count thezero-crossings and electronic circuitry will time this for display orrecording.

The weight cost of this will be small, a few pounds maximum. Additionalspace required will be minimal. The zero-crossing detector and counterare simple electronic devices and will likewise cause little penalty. Anon-board precision oscillator for the tape recorder could also supplythe timing frequency but this too is small and simple. The informationmay be in binary form for direct recording. The power requirement wouldbe a few watts and then only for the period of oscillation.

The foregoing seat arrangement for mass determination is exemplary ofone method. Since man generates three forces, (a) random voluntarymuscular movements which may be consciously controlled and eliminatedfor short periods, (b) cardiovascular forces with fundamentalfrequencies in the order of 1 cycle per second, and (c) respiratorymovements with frequencies in the range of O.2O.3 cycle per second,account must be taken of each force for the errors that may be produced.For example, if the seat is mounted for operation as described relativeto FIG. 2,,

the internal restoring forces and resistance of the subjects body wouldcause an apparent increase in mass with increasing frequency. Tominimize the error, stiffening of the body by contracting themusculature, positioning of the individual and, less desirably.mechanical restraint may be etfected to aid in a control of the periodand amplitude required for measurement. Reduction of the frequency ofoscillation within the limits imposed by the danger of atelectasisduring breath holding in an O atmosphere (15 seconds) for only a fewcycles will give adequate accuracy.

Although the invention has been illustrated in the accompanying drawingsand described in the foregoing specification in terms of a preferredembodiment thereof, the invention is not limited to this embodiment orto the particular uses mentioned. It will be apparent to those skilledin the art that my invention can be practiced utilizing the disclosedmass measurement system for determining the change in mass underlaboratory conditions and for various other reasons. In addition to themeasurement of crew mass, there may be a number of quantitativeprocedures in which a small scale of high precision could provide a massmeasurement much more easily than a volumetric measurement may be made.Also it should be understood that various changes, alterations.modifications, and substitutions, particularly with respect to theconstruction details, can be made in the arrangement of the severalelements without departing from the true spirit and scope of theappended claims, for ex ample, in place of the air bearing and springarrangement for the support structure 13, the support structure could besupported by a vertical leaf spring at each of its two ends such thatsubstantially translatory motion is achieved. The slight curvilinearvariation from true translatory motion for low amplitude oscillationsallows for sufiicient accuracy for most requirements.

Having thus described my invention, what I desire to secure by LettersPatent of the United States is:

1. An apparatus for measuring mass under substan= tially zero gravityconditions, comprising a chair-like support enabling the mass to bemaintained in substantially rigid condition, a spring assemblyoperatively attached between said support and a fixed member, saidspring assembly permitting said mass and support to oscillate as a unit,bearing means to constrain the oscillation of the mass and support alonga single translatory axis parallel to the direction of said springassembly, an optical zero-crossing detector to indicate the frequency ofoscillation of said mass and support, and counter means to indicate eachtime the detector crosses the zero position, said counter meansoperating to determine the period of oscillation, thereby indicating thenatural frequency of the mass and support which varies in proportion tothe change in weight of the mass.

2. The apparatus for measuring mass defined in claim 1 wherein thebearing means for constraining the oscillation of the mass to a singletranslatory axis includes a plurality of air bearings positioned betweenthe support With the attached mass and the fixed member, said airbearings being substantially frictionless under zero gravity conditions.

3. The apparatus for measuring mass defined in claim 1 wherein saidoptical zero-crossing detector includes a light source, a photosensitiveelement in optical alignment with said light source and a knife edgeattached to the mass and positioned tointerrupt the beam from said lightsource each time the mass oscillates thereby causing a correspondinglyintermittent signal to be emitted by said photosensitive element.

4. The apparatus for measuring mass defined in claim 1 wherein saidspring assembly includes a plurality of coil springs having a knownspring constant, at least one of said springs being mounted on saidsupport and attached to the fixed member, another of said springs beingmounted in line with and exerting a force opposite to the first mountedspring, thereby operating to allow said support to oscillate like apendulum when displaced from equilibrium position.

References Cited UNITED STATES PATENTS 2,694,310 11/1954 Pounds 73672,784,588 3/1957 Humble 73-67.2X 2,862,385 12/1958 Woods 737l.63,319,460 5/1967 Barigant 73-672 RICHARD C. QUEISSER, Primary ExaminerJOHN P. BEAUCHAMP, Assistant Examiner U.S. Cl. X.R.

